That seems to be the opinion among some of his Senate colleagues, including those within his own party. Or, if not insane, prone to explosions of rage -- and temperamentally unsuited to the Presidency. Per the WaPo:
"The thought of his being president sends a cold chill down my spine," Sen. Thad Cochran (R-Miss.) ... told the Boston Globe recently. "He is erratic. He is hotheaded. He loses his temper and he worries me."